1. Field of the Invention
The present invention relates to a noise reducing apparatus and an exhaust nozzle for a jet engine, particularly for reducing noise effectively while providing a light weight apparatus.
2. Description of Related Art
In an jet engine, it is known that a large amount of noise occurs due to a jet flow caused by an exhaust gas which is exhausted from an engine. In particular, in a jet engine for a supersonic transporter (SST), there is a large amount of noise because the velocity of jet flow is high; thus, reducing noise which is generated near the ground due to take-off, landing, and low level flight is a problem to be solved.
Therefore, in a jet engine which is used for a supersonic transporter, there has been research on the provision of an exhaust nozzle, for example, as shown in FIG. 9. In FIG. 9, the upper half indicates supersonic cruising flight and the lower half indicates a noise reducing mode. The exhaust nozzle shown in FIG. 9 is provided with a first movable nozzle 101 for controlling flow of an exhaust gas 100 and a second movable nozzle 102 therebehind for mixing an external air with the exhaust gas so as to mix them. A high speed exhaust gas 100 which is exhausted from an engine 103 is accelerated so as to be an ultra-high-speed jet flow by passing a convergent flow channel 104 and a divergent flow channel 105. When noise must be reduced (i.e., during take-off, landing and low level flight), noise reduction is performed by introducing an air outside a second movable nozzle 102 into the second movable nozzle 102, and mixing a high-speed exhaust gas from a first movable nozzle 101 and a low-speed external air by a mixer 106 so as to reduce the speed of the jet flow. On the other hand, during supersonic cruising in which an airplane cruises at a very high altitude where noise is not a serious problem, the introduction of external air is stopped, and jet flow is blown out therebehind by retracting mixer 106.
However, in such an exhaust nozzle for a jet engine, the performance of the mixer is so poor that there was a problem in that noise could not be reduced sufficiently. That is, in order to avoid a deleterious influence on propulsion efficiency due to resistance of a mixer, it is necessary to retract the mixer from the supersonic flow path during cruising. Therefore, there are severe limits on the shape and size of the mixer, it was possible to employ a mixer having only unsatisfactory performance such as a plate-type mixer.
Also, in recent jet engines, there is a trend in which required functions for an exhaust nozzle is increased for the purpose of obtaining optimum propulsion efficiency; thus, research on exhaust nozzles having rectangular cross sections is under way so as to form it for each function, including a noise reducing function, in a relatively a simple structure.
However, in such an exhaust nozzle having a rectangular cross section, it is difficult to achieve high rigidity comparing to an exhaust nozzle having a circular cross section. There is a problem in that the weight of the exhaust nozzle tends to be increased to a great extent by an increased thickness of components during tests for obtaining a required rigidity.